N. Eswara Prasad et al. Engineering Fracture Mechanics 71(2004)2589-2605 DISPLACEMENT (6) g. 3. of hd nosa isee ct ak etah wit d sng me ewa dta in otsr at the rmsisteanae u ihespecckeas ter an e tation length (grven It can be seen from these curves that increase in crack length decreases maximum load attained prior to the commencement of crack extension. The load initially increases linearly with the displacement in all the cases.This corresponds to the stage in which the specimen largely experiences elastic stresses. Followed by his stage, the crack extension takes place. This is reflected in nonlinear increase in the load with dis- placement followed by noticeable drop in the load with further increase in the displacement. The load- displacement curves show distinctly different characteristics at this stage of crack extension in the two notch orientations The material in the crack divider orientation shows a steep, but continuous fall in the load with increase in the displacement(Fig. 2). Such a behaviour is seen in specimens with lower crack lengths(a) and(b)in Fig. 2 with a/W=0.32 and 0.44, respectively). This clearly indicates gradual extension of the crack front However, higher crack length specimens in this notch orientation((c)and(d)in Fig. 2 with a/W=0.56 and 0.65, respectively) do not show such steep load drop. Instead, these specimens show near-saturation in the variation of load with displacement, up to a displacement of 800 um. In this case, the fibre bundles undergo significant bending without breakage and crack extension essentially occurs along fibre/matrix interface. On the other hand, the specimens in the crack arrester orientation show distinct and sudden load drops with crack extension(shown as A1 B1, A2B2 etc, in the curves(a),()and(c)of Fig 3). Again, at large crack lengths(a/W=0.64, case(c)in Fig 3)the specimen shows gradual load drop with displacement Such a behaviour is attributable to the change in the nature of crack extension. As shown schematically in ne specimens in the crack arrester orientation show complete mode I(tensile) fracture dominant fibre bundle breakage leading to relatively insignificant crack extension along the fibre/matrix interface when the crack lengths are smaller(a/w<0.45). As crack length increases, the extent of crack extension along the fibre/matrix interface also increases, leading to significant extent of'H'and'Tcracking (Fig 4b). This results in mode I crack extension in the initial stages and a mixed mode fracture in the later stages, comprising mode I and mode II (in-plane shear or sliding) components. This occurs in case of specimens with a/W values in the range 0.55-0.6. At still higher crack lengths(cases(c)and(d)in Fig. 2 and (c)in Figs. 3 and 4), the crack extension occurs essentially in mode II with interply shearing being theIt can be seen from these curves that increase in crack length decreases maximum load attained prior to the commencement of crack extension.The load initially increases linearly with the displacement in all the cases.This corresponds to the stage in which the specimen largely experiences elastic stresses.Followed by this stage, the crack extension takes place.This is reflected in nonlinear increase in the load with dis￾placement followed by noticeable drop in the load with further increase in the displacement.The load– displacement curves show distinctly different characteristics at this stage of crack extension in the two notch orientations. The material in the crack divider orientation shows a steep, but continuous fall in the load with increase in the displacement (Fig.2).Such a behaviour is seen in specimens with lower crack lengths ((a) and (b) in Fig.2 with a=W ¼ 0:32 and 0.44, respectively). This clearly indicates gradual extension of the crack front. However, higher crack length specimens in this notch orientation ((c) and (d) in Fig.2 with a=W ¼ 0:56 and 0.65, respectively) do not show such steep load drop. Instead, these specimens show near-saturation in the variation of load with displacement, up to a displacement of
800 lm.In this case, the fibre bundles undergo significant bending without breakage and crack extension essentially occurs along fibre/matrix interface.On the other hand, the specimens in the crack arrester orientation show distinct and sudden load drops with crack extension (shown as A1B1, A2B2 etc., in the curves (a), (b) and (c) of Fig. 3). Again, at large crack lengths (a=W ¼ 0:64, case (c) in Fig.3) the specimen shows gradual load drop with displacement. Such a behaviour is attributable to the change in the nature of crack extension.As shown schematically in Fig.4a, the specimens in the crack arrester orientation show complete mode I (tensile) fracture with pre￾dominant fibre bundle breakage leading to relatively insignificant crack extension along the fibre/matrix interface when the crack lengths are smaller (a=W < 0:45).As crack length increases, the extent of crack extension along the fibre/matrix interface also increases, leading to significant extent of ‘H’ and ‘T ’ cracking (Fig.4b).This results in mode I crack extension in the initial stages and a mixed mode fracture in the later stages, comprising mode I and mode II (in-plane shear or sliding) components.This occurs in case of specimens with a=W values in the range 0.55–0.6. At still higher crack lengths (cases (c) and (d) in Fig. 2 and (c) in Figs.3 and 4), the crack extension occurs essentially in mode II with interply shearing being the Fig.3.Load–displacement data obtained during the evaluation of fracture resistance using specimens with varied crack length (given in terms of the normalised crack length with specimen width) in case of the material in the ‘‘crack arrester’’ orientation. N. Eswara Prasad et al. / Engineering Fracture Mechanics 71 (2004) 2589–2605 2593